Simple mail transfer protocol smtp
1 / 29

Simple Mail Transfer Protocol (SMTP) - PowerPoint PPT Presentation

  • Uploaded on

Simple Mail Transfer Protocol (SMTP). Kanika Thapar CISC 856 TCP/IP and Upper Layer Protocols 11/8/2007 (Some slides provided by Ezra Kissel, some figures taken from Forouzan’s book). Overview. Introduction Sendmail How SMTP works? SMTP data transfer Examples Limitations & extensions

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about ' Simple Mail Transfer Protocol (SMTP)' - verda

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Simple mail transfer protocol smtp

Simple Mail Transfer Protocol(SMTP)

Kanika Thapar

CISC 856 TCP/IP and Upper Layer Protocols


(Some slides provided by Ezra Kissel, some figures taken from Forouzan’s book)


  • Introduction

  • Sendmail

  • How SMTP works?

  • SMTP data transfer

  • Examples

  • Limitations & extensions

  • Retrieving mail (Mailbox protocols)

Collects mail and delivers to user agent

Transfers mails from one host to another


Transfers mails from one host to another

Collects mail and delivers to user agent

Composes, reads,

replies to

,forwards and handles mailboxes

Message transfer…

SMTP is a push protocol


  • Mail Transfer Agent [MTA] : is a computer program or software agent that transfers electronic mail messages from one computer to another.

  • Sendmail is a MTA

  • Supports several mail transfers including SMTP

  • Pro’s

    • Can perform header rewriting, mail routing

    • Extensive support available

  • Con’s

    • Not secure

    • Code is bulky [compared to other MTA’s such as qmail]

SMTP server can also assume the role of a “relay”

SMTP mail gateways are used to transport mail prepared by a protocol other than SMTP

Relays and Gateways

Non-SMTP supported

SMTP supported

How SMTP works?

Command format:

Keyword: argument(s)

Response format:

3-digit status code [textual information]

Link Layer PCI

Link Layer PCI






SMTP command/response

SMTP command/response

SMTP command/response


The Basics

The Extras

How SMTP works : Status Codes

The Server responds with a 3 digit code that may be followed by text info

  • 2## - Success

  • 3## - Command can be accepted with more information

  • 4## - Command was rejected, but error condition is temporary

  • 5## - Command rejected, Bad User!

Traditional mail vs email

E-mail envelope and body

Traditional mail envelope and body

Connection establishment
Connection Establishment



Ephemeral port

Ephemeral port

Port 25

Port 25


Connection termination
Connection Termination

Ephemeral port

Port 25




Simulating mta client using telnet
Simulating MTA client using telnet

% telnet 25Trying to (

================== ConnectionEstablishment================

220 SMTP server ready Thur, 8 Nov 2007 ..HELO mail.adelphia.net250


MAIL FROM: [email protected] Sender <[email protected]> OkRCPT TO: [email protected]

250 Recipient <[email protected]> Ok

Simulating mta client using telnet1
Simulating MTA client using telnet

=================== Header and Body ===================

DATA354 Ok Send data ending with <CRLF>.<CRLF>From: ForouzanTO: ThaparThis is a test messageto show SMTP in action..

250 Message received: [email protected]

============= Connection Termination====================


221 SMTP server closing connection

Connection closed by foreign host.

Limitations in smtp
Limitations in SMTP

  • Only uses NVT 7 bit ASCII format

    • How to represent other data types?

  • Susceptible to misuse (Spamming,

    faking sender address)

Solution smtp extensions
Solution: SMTP extensions

  • MIME – Multipurpose Internet Mail Extensions

    Transforms non-ASCII data to NVT (Network Virtual Terminal) ASCII data

Mime headers cont d
MIME headers (cont’d)

  • Content-Type – Type of data used in the Body

    • Text: plain, unformatted text; HTML

    • Multipart: Body contains different data types

    • Message: Body contains a whole, part, or pointer to a message

    • Image: Message contains a static image (JPEG, GIF)

    • Video: Message contains an animated image (MPEG)

    • Audio: Message contains a basic sound sample (8kHz)

    • Application: Message is of data type not previously defined

  • Content-Transfer-Encoding – How to encode the message

    • 7 bit – no encoding needed

    • 8 bit – Non-ASCII, short lines

    • Binary – Non-ASCII, unlimited length lines

    • Base64 – 6 bit blocks encoded into 8-bit ASCII

    • Quoted-printable – send non-ASCII characters as 3 ASCII characters

Base64 encoding
Base64 Encoding

  • Divides binary data into 24 bit blocks

  • Each block is then divided into 6 bit chunks

  • Each 6-bit section is interpreted as one character

11001100 10000001 00111001

110011 001000 000100 111001













Multipart, Encoded MIME Message

From: Kanika Thapar <[email protected]>

To: [email protected]

Subject: attachment test

MIME-Version: 1.0

Content-Type: MULTIPART/MIXED; boundary="MIMEStream=_0+92061_793033260215529_597673089"


Content-Type: TEXT/PLAIN; charset="US-ASCII"; format=flowed

There is an image attached...


Content-Type: IMAGE/jpeg; name="test.jpg"

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename="test.jpg"





Mail access protocols pop3 imap4
Mail Access Protocols POP3 & IMAP4





Post Office Protocol v3

  • Allows the user to obtain a list of their Emails

  • Users can retrieve their emails

  • Users can either delete or keep the email on their system

  • Minimizes server resources

Internet Mail Access Protocol v4

  • User can check the email header before downloading

  • Can search the email for a specific string of characters before downloading

  • User can download parts of an email

  • User can create, delete, or rename mailboxes on a server

Quoted printable encoding
Quoted-Printable Encoding

  • Used when the data has a small non-ASCII portion

  • Non-ASCII characters are sent as 3 characters

  • First is ‘=‘, second and third are the hex representation of the byte

  • =##, ## is the hex representation of the byte

01001100 10011101 00111001